This invention relates generally to electric motors, and more specifically to a system for controlling the speed of a direct-current motor by an external synchronizing signal so as to maintain the same at synchronous speed in spite of possible rapid changes in the load or in the frequency of the synchronizing signal.
As is well known, synchronous motors have a distinct advantage over other types of motors, in that their speed of rotation is easily controllable in step with the frequency of an AC supply system to which they are connected. However, the synchronous motors are difficult of self-starting and, moreover, are forced to stop when they fall out of synchronism due to a rapid change in the load or in the AC supply frequency.
These disadvantages of the synchronous motors are totally non-existent in direct-current motors, which have no substantial starting problems and which do not stop in spite of rapid changes in the load or in the supply voltage. However, the direct-current motors have their own drawback in connection with the difficulties involved in their speed control.